UPDATE 2-Shares in WPP slump on weak 2017 results and nervous 2018 outlook

* 2017 net sales down 0.9 percent

* Forecasts 2018 net sales to be flat

* Shares down 10 percent

* To simplify group to better support clients (Adds quotes, reaction)

LONDON, March 1 (Reuters) - WPP, the world's biggest advertising group, plans to break down the barriers between its agencies to better support clients after it reported its worst annual performance since the financial crisis and forecast no growth for 2018.

The news capped a rough year for WPP, which cut its sales outlook three times. Analysts had hoped it would join its peers Omnicom and IPG in sounding more upbeat about 2018 but the muted outlook sent its shares down 10 percent.

"2017 for us was not a pretty year," the company's founder and Chief Executive Martin Sorrell said.

The British company employs more than 200,000 people in 112 countries through major agencies such as JWT, Ogilvy & Mather and Finsbury that provide advertising services, research, public relations and data analysis.

But the group has been hit this year by some of its biggest clients in the consumer goods sector like Unilever cutting spending and by the encroachment of Google, Facebook and consultants such as Accenture on to its territory.

It reported a 0.9 percent drop in 2017 net sales, after predicting a broadly flat outcome in October, with demand particularly weak in North America. Britain was one of the strongest performers.

For 2018 it said the group's budgets were being set on the basis it would see flat growth for revenue and net sales, with the headline operating margin also flat in constant currency.

Net sales were down 1.2 percent in January.

To counter the pressures on the group, WPP said it would accelerate a programme to simplify the business by aligning digital systems, platforms and capabilities.

A strategy it currently uses for its biggest clients such as Ford, where it creates a single team to provide all the services required by that company, rather than through its multiple agencies, could be rolled out more widely.

"As our industry continues to undergo fundamental change, we are upping the pace of WPP's development from a group of individual companies to a cohesive global team dedicated to the core purpose of driving growth for clients." (Reporting by Kate Holton Editing by Paul Sandle/Keith Weir)